diff --git a/src/main/runtime/rpc/unix-socket-transport.test.ts b/src/main/runtime/rpc/unix-socket-transport.test.ts
index e321c2c59e9..c3d049571aa 100644
--- a/src/main/runtime/rpc/unix-socket-transport.test.ts
+++ b/src/main/runtime/rpc/unix-socket-transport.test.ts
@@ -1,4 +1,5 @@
 import { EventEmitter } from 'node:events'
+import { StringDecoder } from 'node:string_decoder'
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 import type { Socket } from 'node:net'
 import { UnixSocketTransport } from './unix-socket-transport'
@@ -11,6 +12,7 @@ class FakeSocket extends EventEmitter {
   setEncoding(): void {}
   setNoDelay(): void {}
   setTimeout(): void {}
+  end(): void {}
 
   write(data: string): boolean {
     this.writes.push(data)
@@ -40,6 +42,47 @@ describe('UnixSocketTransport', () => {
     vi.useRealTimers()
   })
 
+  function createReceiver() {
+    const transport = new UnixSocketTransport({ endpoint: 'test-pipe', kind: 'named-pipe' })
+    const socket = new FakeSocket()
+    const received: string[] = []
+    transport.onMessage((message, reply) => {
+      received.push(message)
+      reply('ok')
+    })
+    ;(transport as unknown as UnixSocketTransportInternals).handleConnection(
+      socket as unknown as Socket
+    )
+    return { socket, received }
+  }
+
+  it.each([false, true])('preserves the UTF-8 byte boundary with oversized=%s', (oversized) => {
+    const { socket, received } = createReceiver()
+    const message = `${'é'.repeat(524287)}a${oversized ? 'x' : ''}`
+    const wire = Buffer.from(`${message}\n`)
+    const decoder = new StringDecoder('utf8')
+    for (let offset = 0; offset < wire.length; offset += 4095) {
+      socket.emit('data', decoder.write(wire.subarray(offset, offset + 4095)))
+    }
+    expect(received).toEqual([oversized ? '' : message])
+  })
+
+  it('retains only the byte count of the partial tail between messages', () => {
+    const { socket, received } = createReceiver()
+    const large = 'a'.repeat(700000)
+    socket.emit('data', `${large}\npart`)
+    socket.emit('data', `ial\r\n\n${large}\n`)
+    expect(received).toEqual([large, 'partial', large])
+  })
+
+  it('checks the combined incoming buffer before dispatching any complete messages', () => {
+    const { socket, received } = createReceiver()
+    socket.emit('data', `${'a'.repeat(700000)}\n${'b'.repeat(700000)}\n`)
+    expect(received).toEqual([''])
+    socket.emit('data', 'later\n')
+    expect(received).toEqual([''])
+  })
+
   it('clears request keepalive timers when the socket closes before a reply', () => {
     const transport = new UnixSocketTransport({
       endpoint: '/tmp/orca-runtime-rpc-test.sock',
diff --git a/src/main/runtime/rpc/unix-socket-transport.ts b/src/main/runtime/rpc/unix-socket-transport.ts
index ed66f94d907..092d67a0dc9 100644
--- a/src/main/runtime/rpc/unix-socket-transport.ts
+++ b/src/main/runtime/rpc/unix-socket-transport.ts
@@ -105,6 +105,7 @@ export class UnixSocketTransport implements RpcTransport {
   private handleConnection(socket: Socket): void {
     this.activeSockets.add(socket)
     let buffer = ''
+    let retainedBytes = 0
     let oversized = false
     // Why: each in-flight dispatch registers its own AbortController here so
     // `socket.on('close')` can abort them all at once. Keeping the set scoped
@@ -134,10 +135,12 @@ export class UnixSocketTransport implements RpcTransport {
         return
       }
       buffer += chunk
+      // setEncoding('utf8') keeps split codepoints intact, so chunk byte lengths add exactly.
+      retainedBytes += Buffer.byteLength(chunk, 'utf8')
       // Why: the Orca runtime lives in Electron main, so it must reject
       // oversized local RPC frames instead of letting a local client grow an
       // unbounded buffer and stall the app.
-      if (Buffer.byteLength(buffer, 'utf8') > MAX_RUNTIME_RPC_MESSAGE_BYTES) {
+      if (retainedBytes > MAX_RUNTIME_RPC_MESSAGE_BYTES) {
         oversized = true
         this.messageHandler?.('', (response) => {
           socket.write(`${response}\n`)
@@ -145,6 +148,9 @@ export class UnixSocketTransport implements RpcTransport {
         })
         return
       }
+      if (!chunk.includes('\n')) {
+        return
+      }
       let newlineIndex = buffer.indexOf('\n')
       while (newlineIndex !== -1) {
         const rawMessage = buffer.slice(0, newlineIndex).trim()
@@ -154,6 +160,7 @@ export class UnixSocketTransport implements RpcTransport {
         }
         newlineIndex = buffer.indexOf('\n')
       }
+      retainedBytes = Buffer.byteLength(buffer, 'utf8')
     })
   }